MOSCOW - European leaders met into the early hours of Thursday in a grinding, last-ditch effort to forge a cease-fire in Ukraine's increasingly violent year-old conflict, which has brought warfare not seen in years to Europe.

Amid some of the heaviest fighting to date in eastern Ukraine, the leaders of Ukraine, Russia, Germany and France were haggling over a deal to stop the fighting between pro-Russian rebels and pro-government forces. Top officials described the extraordinary summit as the final chance to avert an even harsher escalation.

World is waiting

"The entire world is waiting to see whether the situation moves toward de-escalation, weapons pullback, cease-fire, or spins out of control," Ukrainian President Petro Poroshenko said in the Belarusian capital, Minsk, ahead of the negotiations. He told his Cabinet that he was poised to declare martial law if the talks failed.

Leaders have said they were working from the outlines of a September cease-fire deal that was never fully implemented and that evaporated completely in recent weeks. The death toll has more than doubled since then, to more than 5,400, according to U.N. estimates, and rebels have captured hundreds of square miles of additional territory.

In rough terms, any deal will require both sides to stop shooting and for heavy weaponry to be pulled back from the front lines within days to create a demilitarized zone. Rebel-held territories have demanded deep autonomy from the Ukrainian central government in Kiev.

Points of contention included what form of autonomy and how to define the borders of rebel-held territory, as the battle lines have moved sharply since last summer.

Kiev has also demanded that Russia stop the cross-border flow of weapons and fighters into Ukraine.

The conflict has brought relations between Russia and the West to lows not seen since the Cold War. Within Ukraine, Poroshenko has been forced to give up dreams of leading a unified nation under the full control of Kiev. He has set up hard internal borders and cut off people, goods and money flowing between Kiev-held and rebel-held territories.

But European leaders have tempered expectations about the outcome.

Only a ray of hope

"A ray of hope, but not more," said Steffen Seibert, a spokesman for German Chancellor Angela Merkel, in Berlin before the talks started.

Russian Foreign Minister Sergei Lavrov said late Wednesday that the talks were going well but that a deal may not be ready to be signed until Thursday morning.

Russia wants guarantees that Ukraine will not tilt westward and join alliances such as NATO. Russian President Vladimir Putin said fears of that possibility drove his annexation of Ukraine's Crimean Peninsula last March, a decision that set off the new conflict between Russia and the West.

Anger in the West toward Russia has grown since the fighting reignited last month, and the White House has been considering whether to send arms to Ukraine's military.